In radio reception using heterodyning in the tuning process, the “image frequency” is an undesired input frequency capable of producing an intermediate frequency (“IF”) similar to that of the desired input frequency. It is a potential source of interference to proper reception. Accordingly, achieving good image rejection (“IR”) in heterodyne receivers is one of the most important challenges in high-performance radio-frequency (“RF”) design, and, as a result, the choice of radio architecture used in many applications is often dictated by an application's overall IR requirements. One possible radio architecture uses a zero-IF receiver, which has no image component requiring rejection. The zero-IF architecture is, however, prone to DC offset problems and to low-frequency impairments, such as 1/f noise. These problems render the zero-IF architecture unsuitable for narrowband wireless communication applications, narrowband wireless telemetry, and wireless sensor applications. For example, narrowband wireless communication applications, such as those using the Flex/ReFlex pager and PMR radio standards, require low-frequency occupied spectral bandwidths of 6.25 kHz, 12 kHz, and 25 kHz. Similarly, various regulatory agencies (e.g., FCC in the United States, ETSI in Europe, and ARIB in Japan) permit narrowband wireless telemetry only in selected RF bands (for example, 6.25-25 kHz in the United States and 12.5-25 kHz in Europe and Japan). A zero-IF receiver architecture may be unsuitable for these low-frequency, narrowband applications.
Another radio architecture uses a double superheterodyne to achieve good IR performance. In this architecture, the use of a high first IF frequency may relax the constraints on the RF band-select filter at the low noise amplifier (“LNA”) input and thereby improve IR performance. High-IF receivers, however, generally require expensive and power-hungry filters for the first IF stage, rendering them unsuitable for low-power applications.
A double-superheterodyne receiver may use a low first IF frequency to relax the bandwidth, power, and cost constraints on the first IF filter. These receivers, however, require a sharper RF band-select filer at the LNA input. A low-IF receiver architecture overcomes the low-frequency and 1/f noise problems of the zero-IF receiver by moving the received spectrum away from DC. As a result, this receiver architecture is more suitable for the narrowband wireless telemetry applications described above. Unlike the zero-IF receiver, however, a low-IF receiver includes a complex mixer or poly-phase filter to reject the generated image frequency. In general, traditional low-IF receivers rely on complex signal cancellation techniques to remove the image component. Due to manufacturing process tolerances, however, it is difficult to ensure quadrature gain and phase errors of better than 1-2% and 1-3 degrees, respectively, which results in a typical image rejection performance of 25-30 dB.
Some low-IF receivers improve their image rejection performance with image-calibration circuitry, which attempts to compensate for the gain and phase errors caused by manufacturing process tolerances in the receiver's components. Conventional calibration circuits, however, suffer from numerous drawbacks. Phase and/or gain adjust circuits that operate at RF frequencies consume a significant amount of power and are difficult to design with a wide dynamic range. Digital phase and/or gain adjust circuits may be implemented to operate at baseband or IF frequencies, but they typically require at least four multipliers to operate—with associated area and power penalties—and incur an inherent loss in precision due to the digitization of the analog RF signal. Thus, there is a need for a robust, low-cost, and low-power image-calibration circuit for RF receivers.